The present invention relates generally to wireless radio-frequency (RF) communication systems. In particular, it relates to isolation systems that separate a desired receive signal from interfering signals in a RF communication system. Interfering signals include all signals received by a receiving antenna that cause co-channel interference.
Some techniques for reducing co-channel interference include frequency separation, time division, orthogonal polarization, and spatial separation. Further reduction of interference requires some type of cancellation. U.S. Pat. No. 5,432,522 shows a canceller that reduces cross-polarization interference in two orthogonally polarized channels. U.S. Pat. No. 5,515,378 shows how adaptive beamforming performed with a phased array provides spatial multiplexing and demultiplexing of wireless communication channels. Each element of the array has an associated electrical signal that is adjusted by a complex-valued weight, then summed to provide an antenna beam pattern having nulls (canceled responses) in a predetermined direction for a particular signal frequency. Transmission of wide-band or multiple-frequency signals causes distortion of the main beam and variance in the location and depth (beam-pattern magnitude) of the nulls.
Beamforming can provide exceptional performance in a fading environment, which is due to the ability of an array to select signals based on the signals directions of arrival. As strong signals are selected for reception, destructive cancellation caused by reflected components arriving at the array elements is mitigated by the placement of nulls. Null placement is also effective in mitigating co-channel interference. However, problems with beamforming include the inability to resolve co-located or closely spaced radio sources unless multipath components of these signals are tracked. In addition, the number of antenna elements limits the number of co-channel interference sources that can be nulled. This is a significant problem because each multipath component arriving at the array is a source of interference. Therefore, a small number of transmitters may provide a large number of interference sources.